1. Field of the Invention
The present invention generally relates to systems for controlling continuous sheetmaking systems and, more specifically, to sensors for measuring the fiber weight of wetstock in a papermaking machine.
2. State of the Art
In the art of modern high-speed papermaking, it is well known to continuously measure certain properties of the paper material in order to monitor the quality of the finished product. These on-line measurements often include basis weight, moisture content, and sheet caliper (i.e., thickness). The measurements can be used for controlling process variables with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to upsets in the manufacturing process.
The on-line sheet property measurements are often accomplished by scanning sensors that periodically traverse the sheet material from edge to edge. For example, a high-speed scanning sensor may complete a scan in a period as short as twenty seconds, with measurements being read from the sensor at about 50 milliseconds intervals. It is also know that a series of stationary sensors can be used to make similar on-line measurements.
In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers complete the drying process. The paper machine is, in essence, a de-watering, i.e., water removal, system. A typical forming section of a papermaking machine includes an endless traveling papermaking mesh fabric or wire which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. As the material travels on the mesh fabric over the series of water removal elements, there is a distinct line of demarcation showing a change in the state of the stock from an extremely wet state to a relatively dryer state. This visible line of demarcation (referred to as the dry line) is characterized in that one side of the dry line has a glossy appearance (i.e., wet state) and the other side of the line has a non-glossy appearance (i.e., relatively dry state). The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper. Other, papermaking devices well known in the art are described for example in U.S. Pat. No. 5,400,258.
Many factors influence the rate at which water is removed which ultimately affects the quality of the paper produced. As is apparent, it would be advantageous to monitor the dynamic process so as to, among other things, predict and control the dry stock weight of the paper that is produced.
It is conventional to measure the moisture content on leaving the main dryer section or at the take up reel employing scanning sensors. Such measurement may be used to adjust the machine operation toward achieving desired parameters. One technique for measuring moisture content is to utilize the absorption spectrum of water in the infra-red. Monitoring or gauge apparatus for this purpose is commonly in use. Such apparatus conventionally uses either a fixed gauge or a gauge mounted on a scanning head which is repetitively scanned transversely across the web at the exit from the dryer section and/or upon entry to the take up reel, as required by the individual machines. The gauges typically use a broad-band infra-red source and one or more detectors with the wavelength of interest being selected by a narrow-band filter, for example, an interference type filter. The gauges used fall into two main types: the transmissive type in which the source and detector are on opposite sides of the web and, in a scanning gauge, are scanned in synchronism across it, and the scatter type (sometimes called xe2x80x9creflectivexe2x80x9d type) in which the source and detector are in a single head on one side of the web, the detector responding to the amount of source radiation scattered from the web.
The present invention, in general, is a sensor imbedded roller which, in one embodiment, is used in a measurement apparatus in a sheetmaking machine. In a preferred embodiment, the measurement apparatus includes a fixed impedance element coupled in series with a detection cell in the sensor which is coupled between an input signal and a reference potential (e.g., ground) and which has a variable impedance. The fixed impedance element and the detection cell form a voltage divider network such that changes in impedance of the detection cell results in changes in voltage on the output of the measurement system. The impedance of the detection cell represents the impedance of the physical configuration of electrodes within the sensor and the material residing between and in close proximity to the electrodes. The impedance relates to the property of the material being measured.
In one embodiment, the measurement apparatus is used to measure the conductivity of an aqueous mixture (referred to as wetstock) in a sheetmaking system. The conductivity of the wetstock is directly proportional to the total water weight within the wetstock, consequently providing information which can be used to monitor and control the quality of the paper sheet produced by the papermaking system. In this embodiment the sensor imbedded roller is positioned in the wet end of the sheetmaking system so as to detect conductivity changes.
In another embodiment, the measurement apparatus is used to measure the weight of dry paper sheet in a sheetmaking machine. In this application, the conductivity is negligible and the capacitive impedance is inversely proportional to the dielectric constant and the amount of paper between the electrodes of the measurement apparatus. In this embodiment the sensor embedded roller is positioned in the dry end of the sheetmaking system so as to detect capacitive impedance changes.
The fixed impedance element can be embodied as a resistor, capacitor, inductor or a combination of the three and the input signal is an analog signal. In the embodiment in which the impedance element is an inductor, the impedance of the inductor can be selected to be a particular magnitude by setting the frequency of the input signal so that the impedance of the fixed impedance element can be set to the same range as the impedance of the sensor for optimum sensor sensitivity. Hence, in the case in which the impedance of the sensor varies due to fluctuations in operating conditions of the system or the material being sensed, the impedance of the inductor can be customized to match the sensor impedance without any hardware changes.
The sensor comprises an array of electrodes having a particular configuration which forms measurement cells, each cell being independently coupled to an input signal provided by a signal generator through the impedance element. In a preferred embodiment used to detect the conductivity of an aqueous fibrous mixture, the impedance elements are implemented as resistive elements. Each cell forms a voltage divider network made-up of the resistive element coupled between the signal generator and portions or segments of electrodes within a given cell and of a resistance resulting from the effective water resistance between the electrode portions and segments. The output of each cell is taken from an electrode segment or portion, i.e., the point between the resistive element and the cell. As the conductance of the aqueous mixture changes so does the output voltage of the cell. The output voltage of each cell is coupled to a detector which, in one embodiment, includes circuitry for enhancing the signal such as an amplifier for amplifying the output signal from each cell and a rectifier. In one embodiment of the present invention the detector includes circuitry for converting the output voltages from each cell into data relating to the weight of the aqueous mixture or to other aqueous mixture characteristics.
A first embodiment of the sensor electrode configuration includes first and second elongated segmented side electrodes and a center elongated electrode spaced-apart and centered between the side electrodes all in essentially the same plane. Segments in the two side electrodes are configured such that the segments in the first segmented electrode are staggered with respect to segments in the second segmented electrode. A cell within the first embodiment array configuration is defined as including one of the segments and a corresponding portion of the center electrode opposite to that segment. In a second embodiment, the first embodiment sensor array includes additional grounded electrodes situated along side each of the first and second segmented electrodes so as to guard against current leakage to conductors in the vicinity of the electrodes.
A third embodiment of the sensor electrode configuration includes two elongated grounded side electrodes and a center segmented elongated electrode spaced-apart and centered between the side electrodes. A cell within the third embodiment electrode configuration is defined as including one of the electrode segments in the center electrode and the corresponding portion of the grounded side electrodes situated adjacent to and opposite of this center electrode segment. The fourth embodiment of the electrode configuration is a variation of the third electrode configuration in that it includes a single grounded elongated electrode adjacent a single elongated segmented electrode.
In a preferred embodiment, the apparatus includes a feedback circuit which is used to adjust the input signal provided from the signal generator to compensate for changes in properties of the material that is not being sensed, but that also may affect the output voltages of the cells.
In a preferred embodiment, the measurement apparatus is used in a sheetmaking system which includes a web and at least one sensor electrode configuration imbedded into a roller in a sheetmaking system. As the roller turns, one of the electrode arrays comes in contact with the pre-formed paper material. When in contact, the array provides a read out giving an instantaneous cross-directional (CD) measurement. The sensor imbedded rollers can be mounted anywhere down the machine including the reel location. In one embodiment, the roller is positioned beneath the web and is driven at the same speed as the web such that the point of contact of an electrode array and the web does not move thereby eliminating sliding friction, wear on the array, and residue build-up on the array.
In another embodiment, more than one (CD) electrode array is imbedded in the circumference of the roll so as to provide more than one CD measurement as the roll rotates and each of the imbedded arrays come in contact with the web. In still another embodiment, four CD arrays are imbedded in the roll such that four measurements are provided for each revolution of the roll.
In one embodiment, the sensor imbedded roller includes a stationary center portion covered by a rotatable shell disposed about the center portion. At least one sensor is imbedded into the outer surface of the rotatable shell. The imbedded sensor on the outer surface of the rotatable shell is electrically connected to electrical contacts on the inner surface of the shell. Wires from the remainder of a measurement apparatus for coupling to the imbedded sensor are coupled to a sliding contact assembly attached to the stationary core portion. Electrical contact is made with outer surface imbedded sensors as the inner surface contacts of the rotatable shell pass over the sliding contact assembly. Each sensor in the shell uses the same sliding contact assembly such that only one is in contact with it at a time so as to detect property changes of the material being sensed.
In another embodiment, the sensor imbedded roller includes processing circuitry, and in one embodiment, the processing circuitry serializes measurement data received from the sensor to minimize the number of external wire connections coming from the roller.
In a preferred embodiment, the sensor embedded roller is implemented with a metal surface or a metal roll imbedded with dielectric insulating layers and metal layers to form the electrode array. In one particular embodiment, the metal is steel.
In still another embodiment, the paper sheet is rolled around the roller so as to prolong sensor and material contact to provide an extended measurement time.